There are known internal combustion engines each of which has a plurality of fuel injection modes. As one example of such internal combustion engines, the one that includes a port injection valve and a cylinder injection valve and changes an injection ratio from each of the injection valves is cited, as described in, for example, Japanese Patent Laid-Open No. 2009-257192. Further, an internal combustion engine that can change the number of times of injection in a port injection type internal combustion engine is cited as one of such internal combustion engines.
In the internal combustion engine having a plurality of injection modes, an optimal injection mode is determined in accordance with the operation states such as an engine speed and a load. When the injection mode is changed, the calculation method of the fuel injection amount is also changed in response thereto. This is because the easiness of vaporization and advancement of vaporization of an injected fuel differ in accordance with the injection mode. For example, in the case of cylinder injection, the fuel injection amount can be determined on the assumption that most of the fuel injected from the fuel injection valve is provided for combustion. In contrast with this, in the case of port injection, the fuel injection amount needs to be determined with consideration given to the ratio of the amount of the fuel that adheres to the wall surface of the port to the fuel injection amount, and the ratio of the amount of the vaporized fuel to the adhering fuel amount. The fuel injection amounts are calculated by the methods corresponding to the injection modes like this, and thereby, control precision of the air-fuel ratio can be kept, no matter what injection mode is selected.
However, concerning the return time from fuel cut, the control precision of the air-fuel ratio cannot be always kept with the conventional control method for an internal combustion engine. During implementation of fuel cut, phenomena occur, such as a decrease of an adhering fuel by being taken out by air, and reduction of temperature of the valve and the wall surface, which do not occur during fuel injection. As a result, before fuel cut, and at a return time from the fuel cut, parameters for use in calculation of the fuel injection amount significantly change. In the conventional control method for an internal combustion engine, the injection mode is determined as a natural consequence in accordance with the operation conditions, and therefore, there are the possibilities that the injection mode differs at each return from fuel cut, and that the injection mode is changed immediately after return. For example, in the control device described in Japanese Patent Laid-Open No. 2009-257192, the ratio of port injection and the ratio of cylinder injection are changed in response to the operation state at the time of return from fuel cut. If the injection mode differs, the calculation method of the fuel injection amount using the aforementioned parameters also differs, and if the injection mode is changed halfway, the calculation method of the fuel injection amount is further complicated. Therefore, with the conventional control method for an internal combustion engine, there is concern of being incapable of correctly calculating the fuel injection amount necessary to keep the air-fuel ratio optimal in the case of return from fuel cut.